The present invention relates to a local oscillator for generating a local oscillation signal for frequency conversion in a radio frequency receiving terminal (RF Front End) of a radio or satellite communication apparatus that transmits information in an extremely high frequency (EHF) band, and more particularly, to a circuit that suppresses parasitic signals accompanying the local oscillation signal.
Ordinarily, a radio or satellite telecommunication device that uses the extremely high frequency band (above 3 GHz), converts the audio and image information of a baseband frequency into an extremely high frequency band for transmission and converts a high frequency signal of the extremely high frequency band that is received into a baseband. To do this, a local oscillator of the device must generate a local oscillation frequency above a certain number of gigahertz (109/sec.). In addition, a phase-locked oscillator that uses a phase-locked loop is used as a local oscillator device in radio and satellite telecommunication devices that use an extremely high frequency. However, at the phase-locked oscillator output, a parasitic signal appears which is multiplied along with the output signals of a high stability oscillator (such as a crystal oscillator) used as a reference oscillator of the phase-locked loop. When used as a local oscillator for primary frequency conversion of an oscillator RF front end having such a characteristic, the main signal and parasitic signal which include information in the primary intermediate frequency band (ordinarily the 1 GHz band) become amplified (up to approximately 30 db) together in an intermediate frequency amplifier, deteriorating the receiving characteristics to demodulate information.
FIG. 1 shows a circuit of the conventional local oscillator. FIG. 1 shows a multiplier 11, phase comparator 12, low-pass filter 13, and directional coupler 15 used to construct a phase-locked loop for stabilizing a voltage controlled oscillator 14 of the EHF band by the output of a reference oscillator 20 having a certain hundred Mhz oscillation frequency in order to solve the aforementioned problem. After the output is amplified in an amplifier 40, a band pass filter 50 is positioned at the output terminal of the oscillator to pass only the oscillator signal of the required frequency and suppress the parasitic signals. Circuit parameter design of the bandpass filter is set in accordance with the desired characteristics and frequency band and a microstrip line of approximately 30-80 mm is required in order to attain a cut-off characteristic for a 50 db parasitic signal in a 4 GHz or 12 GHz frequency band generally used in satellite telecommunication and the size of the mechanism should be set according to this.
The oscillator illustrated in FIG. 1 is the 10.75 GHz local oscillator used in a receiver of an outdoor VSAT (Very Small Aperture Terminal) system having a 12 GHz band currently in service in the United States.
Here, the oscillation frequency of the reference oscillator 20 is 50 MHz and a high stability quartz crystal oscillator is ordinarily used. A phase-locked loop is used to phase-lock to the reference oscillator 20 in order to improve the degree of stability of a 10.75 GHz voltage controlled oscillator 14. In this process, among the output signals of phase-locked oscillator 10, multiple frequencies of 50 MHz to 1 GHz along with the main 10.75 GHz signal are weakly spread out (-60 dbc, here, dbc is relative signal strength to the main 10.75 GHz signal) to 1 GHz and overlap the 0.95 GHz-1.45 GHz band which is the first intermediate frequency band. As this takes place, the receiving characteristics are deteriorated when restoring information and to solve this, a 10.75 GHz pass band filter 50 is used at the output terminal of an amplifier 40. Composition of the pass band filter 50 is not simple and its size is not a small problem for concern. Therefore, the conventional EHF oscillator parasitic signal suppression circuit is enlarged in size.